1. Field of the Invention
The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser having a super lattice structure near an active layer.
2. Description of the Prior Art
A well-known conventional semiconductor laser using a super lattice of this type has a conduction band having an MQW (Multi-Quantum-Well) structure. FIG. 1 shows a conduction band (CB) typically used in such a semiconductor laser. The constituting materials have three bandgaps. Materials 31, 32 and 33 have bandgaps having sizes which increase in the order of 31, 32 and 33. The material 31 having a largest bandgap in GaAs.Al.sub.x Ga.sub.1-x As systems is, e.g., Al.sub.x Ga.sub.1-x As (e.g., x=0.4) having a large value of X. The material 31 is called a wide gap material with respect to the material 33 (consisting of GaAs) having a minimum bandgap. The material 32 consists of Al.sub.x Ga.sub.1-x As (x.perspectiveto.0.2), and is a wide gap material with respect to the material 33.
A layer 34 is an active/optical confinement layer. In the layer 34, a material (the material 32 in FIG. 1) having a wider bandgap is called a "wide gap material", and a material (the material 33 in FIG. 1) having a narrower bandgap is called a "narrow gap material". The thickness of the two materials are represented by LB and LW, respectively. In the MQW structure, LB is normally about 20 to 40 .ANG., and LW is selected to be 30 to 150 .ANG.. A semiconductor layer having such a structure is known to oscillate at a low threshold current.
FIG. 2 shows a conduction band (CB) of a semiconductor laser having a similar super lattice structure called GRIN-SCH (Graded-Index Waveguide and Separate Carrier and Optical Confinement Heterostructure) structure. Materials 41 and 43 respectively have the same compositions as the materials 31 and 32 in FIG. 1. When a GaAs.AlGaAs system is used, the material 42 has a composition such that x in Al.sub.x Ga.sub.1-x As gradually decreases toward the material or active layer 43. Electrons and holes are recombined mainly in the active layer 43 and emit light. The light emitted by electron-hole recombination is confined in the region of the material 42 mainly by the diffraction effect. Therefore, the structure shown in FIG. 2 is considered to have an oscillation threshold current lower than that of the MQW structure. The structure in FIG. 2 has an active/optical confinement layer 44.
In the above-described conventional semiconductor layer (GRIN-SCH structure, the composition in the optical confinement layer (material 42) must be gradually changed. In order to achieve this, the crucible temperature of a deposition source must be increased/decreased within a short period of time. Temperature control for this has been difficult to actually perform.